Procedures

THIS SUBROUTINE CORRECTS POSITION VECTOR FOR ABERRATION OF LIGHT. ALGORITHM INCLUDES RELATIVISTIC TERMS. ADAPTED FROM MURRAY (1981) MON. NOTICES ROYAL AST. SOCIETY 195, 639-648.

THIS SUBROUTINE CONVERTS A VECTOR TO ANGULAR QUANTITIES.

Normalize angle into the range -pi <= A < +pi.

THIS SUBROUTINE SUPPLIES THE VALUES OF ASTRONOMICAL CONSTANTS.

This subroutine computes calendar date and time, given julian date. input julian date can be based on any ut-like time scale (utc, ut1, tt, etc.) - output time value will have same basis. output calendar date will be gregorian. algorithm by fliegel and van flandern.

THIS SUBROUTINE TRANSFORMS A STAR'S CATALOG QUANTITIES FOR A CHANGE OF EPOCH AND/OR EQUATOR AND EQUINOX. IT CAN ALSO BE USED TO ROTATE CATALOG QUANTITIES ON THE DYNAMICAL EQUATOR AND EQUINOX OF J2000.0 TO THE ICRS OR VICE VERSA.

THIS SUBROUTINE ROTATES A VECTOR FROM THE CELESTIAL TO THE TERRESTRIAL SYSTEM. SPECIFICALLY, IT TRANSFORMS A VECTOR IN THE GCRS (A LOCAL SPACE-FIXED SYSTEM) TO THE ITRS (A ROTATING EARTH-FIXED SYSTEM) BY APPLYING ROTATIONS FOR THE GCRS-TO- DYNAMICAL FRAME TIE, PRECESSION, NUTATION, EARTH ROTATION, AND POLAR MOTION.

THIS SUBROUTINE RETURNS THE ORTHONORMAL BASIS VECTORS, WITH RESPECT TO THE GCRS (GEOCENTRIC ICRS), OF THE CELESTIAL INTERMEDIATE SYSTEM DEFINED BY THE CELESTIAL INTERMEDIATE POLE (CIP) (IN THE Z DIRECTION) AND THE CELESTIAL INTERMEDIATE ORIGIN (CIO) (IN THE X DIRECTION). A TDB JULIAN DATE AND THE RIGHT ASCENSION OF THE CIO AT THAT DATE IS REQUIRED AS INPUT. THE RIGHT ASCENSION OF THE CIO CAN BE WITH RESPECT TO EITHER THE GCRS ORIGIN OR THE TRUE EQUINOX OF DATE -- DIFFERENT ALGORITHMS ARE USED IN THE TWO CASES.

THIS SUBROUTINE RETURNS THE LOCATION OF THE CELESTIAL INTERMEDIATE ORIGIN (CIO) FOR A GIVEN JULIAN DATE, AS A RIGHT ASCENSION WITH RESPECT TO EITHER THE GCRS (GEOCENTRIC ICRS) ORIGIN OR THE TRUE EQUINOX OF DATE. THE CIO IS ALWAYS LOCATED ON THE TRUE EQUATOR (=INTERMEDIATE EQUATOR) OF DATE.

THIS SUBROUTINE COMPUTES THE TRUE RIGHT ASCENSION OF THE CELESTIAL INTERMEDIATE ORIGIN (CIO) AT A GIVEN TT JULIAN DATE. THIS IS -(EQUATION OF THE ORIGINS).

GIVEN AN INPUT TDB JULIAN DATE AND THE NUMBER OF DATA POINTS DESIRED, THIS SUBROUTINE RETURNS A SET OF JULIAN DATES AND CORRESPONDING VALUES OF THE GCRS RIGHT ASCENSION OF THE CELESTIAL INTERMEDIATE ORIGIN (CIO). THE RANGE OF DATES IS CENTERED (AT LEAST APPROXIMATELY) ON THE REQUESTED DATE. THE SUBROUTINE OBTAINS THE DATA FROM AN EXTERNAL DATA FILE.

THIS SUBROUTINE RETURNS THE DIFFERENCE IN LIGHT-TIME, FOR A STAR, BETWEEN THE BARYCENTER OF THE SOLAR SYSTEM AND THE OBSERVER (OR THE GEOCENTER).

THIS SUBROUTINE CONVERTS AN ECLIPTIC POSITION VECTOR TO AN EQUATORIAL POSITION VECTOR.

Equation of the equinoxes complementary terms, consistent with IAU 2000 resolutions.

THIS SUBROUTINE CONVERTS AN EQUATORIAL POSITION VECTOR TO AN ECLIPTIC POSITION VECTOR.

THIS SUBROUTINE CONVERTS RIGHT ASCENSION AND DECLINATION TO ECLIPTIC LONGITUDE AND LATITUDE.

THIS SUBROUTINE CONVERTS ICRS RIGHT ASCENSION AND DECLINATION TO GALACTIC LONGITUDE AND LATITUDE. IT USES THE TRANSFORMATION GIVEN IN THE HIPPARCOS AND TYCHO CATALOGUES, VOL. 1, SECTION 1.5.3.

THIS SUBROUTINE COMPUTES THE INTERMEDIATE RIGHT ASCENSION OF THE EQUINOX AT JULIAN DATE TJD, USING AN ANALYTICAL EXPRESSION FOR THE ACCUMULATED PRECESSION IN RIGHT ASCENSION. FOR THE TRUE EQUINOX THE RESULT IS THE EQUATION OF THE ORIGINS.

THIS SUBROUTINE RETURNS THE VALUE OF THE EARTH ROTATION ANGLE (THETA) FOR A GIVEN UT1 JULIAN DATE. THE EXPRESSION USED IS TAKEN FROM THE NOTE TO IAU RESOLUTION B1.8 OF 2000.

THIS SUBROUTINE COMPUTES QUANTITIES RELATED TO THE ORIENTATION OF THE EARTH'S ROTATION AXIS AT JULIAN DATE TJD.

FOR USE WITH SUBROUTINE SOLSYS VERSION 1. THIS SUBROUTINE MAY BE CALLED TO CHANGE THE VALUES IN COMMON BLOCK SSFILE, WHICH CONTAINS INFORMATION ON THE COORDINATE FILE USED BY SUBROUTINE SOLSYS.

THIS SUBROUTINE TRANSFORMS A VECTOR FROM THE DYNAMICAL REFERENCE SYSTEM TO THE INTERNATIONAL CELESTIAL REFERENCE SYSTEM (ICRS), OR VICE VERSA. THE DYNAMICAL REFERENCE SYSTEM IS BASED ON THE DYNAMICAL MEAN EQUATOR AND EQUINOX OF J2000.0. THE ICRS IS BASED ON THE SPACE-FIXED ICRS AXES DEFINED BY THE RADIO CATALOG POSITIONS OF SEVERAL HUNDRED EXTRAGALACTIC OBJECTS. THE ROTATION MATRIX USED HERE IS EQUIVALENT TO THAT GIVEN BY HILTON AND HOHENKERK (2004), ASTRONOMY AND ASTROPHYSICS 413, 765-770, EQ. (6) AND (8).

THIS SUBROUTINE COMPUTES FUNDAMENTAL ARGUMENTS (MEAN ELEMENTS) OF THE SUN AND MOON. SEE SIMON ET AL. (1994) ASTRONOMY AND ASTROPHYSICS 282, 663-683, ESPECIALLY SECTIONS 3.4-3.5.

THIS SUBROUTINE CONVERTS GCRS RIGHT ASCENSION AND DECLINATION TO COORDINATES WITH RESPECT TO THE EQUATOR OF DATE (MEAN OR TRUE). FOR COORDINATES WITH RESPECT TO THE TRUE EQUATOR OF DATE, THE ORIGIN OF RIGHT ASCENSION CAN BE EITHER THE TRUE EQUINOX OR THE CELESTIAL INTERMEDIATE ORIGIN (CIO).

THIS SUBROUTINE MOVES THE ORIGIN OF COORDINATES FROM THE BARYCENTER OF THE SOLAR SYSTEM TO THE OBSERVER (OR THE GEOCENTER). I.E., THIS SUBROUTINE ACCOUNTS FOR PARALLAX (ANNUAL+GEOCENTRIC OR JUST ANNUAL).

THIS SUBROUTINE COMPUTES THE GEOCENTRIC POSITION AND VELOCITY OF AN OBSERVER ON THE SURFACE OF THE EARTH OR ON A NEAR-EARTH SPACECRAFT. THE FINAL VECTORS ARE EXPRESSED IN THE GCRS.

THIS SUBROUTINE CONVERTS HIPPARCOS DATA AT EPOCH J1991.25 TO EPOCH J2000.0. TO BE USED ONLY FOR HIPPARCOS OR TYCHO STARS WITH LINEAR SPACE MOTION. BOTH INPUT AND OUTPUT DATA IS IN THE ICRS.

THIS SUBROUTINE ALLOWS THE USER TO RETRIEVE THE LAST COMPUTED POSITION ON THE SKY AS A UNIT VECTOR.

THIS SUBROUTINE CORRECTS POSITION VECTOR FOR THE DEFLECTION OF LIGHT IN THE GRAVITATIONAL FIELD OF AN ARBITRARY BODY. ADAPTED FROM MURRAY (1981) MON. NOTICES ROYAL AST. SOCIETY 195, 639-648. SEE ALSO FORMULAE IN SECTION B OF THE ASTRONOMICAL ALMANAC, OR KAPLAN ET AL. (1989) ASTRONOMICAL JOURNAL 97, 1197-1210, SECTION III F. THIS SUBROUTINE VALID FOR AN OBSERVED BODY WITHIN THE SOLAR SYSTEM AS WELL AS FOR A STAR.

SUBROUTINE GRVDEF VERSION 2. THIS SUBROUTINE COMPUTES THE TOTAL GRAVITATIONAL DEFLECTION OF LIGHT FOR THE OBSERVED OBJECT DUE TO THE MAJOR GRAVITATING BODIES IN THE SOLAR SYSTEM. THIS VERSION IS A DUMMY. NO CORRECTION IS APPLIED.

THIS SUBROUTINE COMPUTES THE TOTAL GRAVITATIONAL DEFLECTION OF LIGHT FOR THE OBSERVED OBJECT DUE TO THE MAJOR GRAVITATING BODIES IN THE SOLAR SYSTEM. THIS SUBROUTINE VALID FOR AN OBSERVED BODY WITHIN THE SOLAR SYSTEM AS WELL AS FOR A STAR. SEE KLIONER (2003), ASTRONOMICAL JOURNAL 125, 1580-1597, SECTION 6.

THIS FUNCTION RETURNS THE ID NUMBER OF A SOLAR SYSTEM BODY FOR THE VERSION OF SOLSYS (OR SOLSYS-AUXPOS COMBINATION) IN USE.

FOR USE WITH SOLSYS VERSION 1. THIS FUNCTION RETURNS THE ID NUMBER OF A SOLAR SYSTEM BODY FOR THE VERSION OF SOLSYS (OR SOLSYS-AUXPOS COMBINATION) IN USE. FOR SOLSYS VERSION 1, THE ID NUMBER OF A BODY REFERS TO ITS ORDER WITHIN EACH RECORD OF THE COORDINATE FILE, WITH ID NUMBERS BEGINNING AT 0 FOR THE FIRST BODY (NORMALLY THE SUN).

This subroutine computes julian date, given calendar date and time. input calendar date must be gregorian. input time value can be in any ut-like time scale (utc, ut1, tt, etc.) - output julian date will have same basis. algorithm by fliegel and van flandern.

THIS FUNCTION DETERMINES THE ANGLE OF AN OBJECT ABOVE OR BELOW THE EARTH'S LIMB (HORIZON). THE GEOMETRIC LIMB IS COMPUTED, ASSUMING THE EARTH TO BE AN AIRLESS SPHERE (NO REFRACTION OR OBLATENESS IS INCLUDED). THE OBSERVER CAN BE ON OR ABOVE THE EARTH. FOR AN OBSERVER ON THE SURFACE OF THE EARTH, THIS SUBROUTINE RETURNS THE APPROXIMATE UNREFRACTED ALTITUDE.

THIS SUBROUTINE COMPUTES THE POSITION OF A SOLAR SYSTEM BODY, AS ANTEDATED FOR LIGHT-TIME.

THIS SUBROUTINE COMPUTES THE ICRS POSITION OF A STAR, GIVEN ITS APPARENT PLACE AT DATE TJD. PROPER MOTION, PARALLAX, AND RADIAL VELOCITY ARE ASSUMED TO BE ZERO.

SUBROUTINE NOD VERSION 2. IN LOW-ACCURACY MODE, THIS SUBROUTINE EVALUATES A SHORT NUTATION SERIES AND RETURNS APPROXIMATE VALUES FOR NUTATION IN LONGITUDE AND NUTATION IN OBLIQUITY FOR A GIVEN TDB JULIAN DATE. IN THIS MODE, ONLY THE LARGEST 13 TERMS OF THE IAU 2000A NUTATION SERIES ARE EVALUATED. IN HIGH-ACCURACY MODE, THE STANDARD IERS SUBROUINE IS CALLED TO EVALUATE THE FULL IAU 2000A NUTATION SERIES.

This subroutine returns the values for nutation in longitude and nutation in obliquity for a given TDB Julian date.

Nutation, IAU 2000A model (MHB_2000 without FCN).

Nutation, IAU 2000A model (MHB_2000 without FCN) MODIFIED. Series truncated for speed of execution, and using Simon et al. (1994) fundamental arguments throughout. Accuracy, compared to IAU 2000 A series, is 0.1 mas in delta psi and 0.04 mas in delta epsilon and delta psi sin(epsilon) over 6 centuries centered at year 2000 (99% of errors less than these values).

THIS SUBROUTINE NUTATES EQUATORIAL RECTANGULAR COORDINATES FROM THE MEAN DYNAMICAL EQUATOR AND EQUINOX OF EPOCH TO THE TRUE EQUATOR AND EQUINOX OF EPOCH. SEE EXPLANATORY SUPPLEMENT TO THE ASTRONOMICAL ALMANAC, PP. 114-115.

THIS SUBROUTINE COMPUTES THE APPARENT DIRECTION OF A STAR OR SOLAR SYSTEM BODY AT A SPECIFIED TIME AND IN A SPECIFIED COORDINATE SYSTEM. BASED ON KAPLAN, ET AL. (1989), ASTRONOMICAL JOURNAL 97, 1197-1210, WITH SOME ENHANCEMENTS FROM KLIONER (2003), ASTRONOMICAL JOURNAL 125, 1580-1597.

THE ENTRIES TO THIS SUBROUTINE PROVIDE 'FRONT ENDS' TO SUBROUTINE PLACE, TAILORED TO SPECIFIC PLACE TYPES. THEY PROVIDE COMPATIBILITY WITH PREVIOUSLY SUPPORTED CALLING SEQUENCES.

THIS SUBROUTINE PRECESSES EQUATORIAL RECTANGULAR COORDINATES FROM ONE EPOCH TO ANOTHER. THE COORDINATES ARE REFERRED TO THE MEAN DYNAMICAL EQUATOR AND EQUINOX OF THE TWO RESPECTIVE EPOCHS. SEE EXPLANATORY SUPPLEMENT TO THE ASTRONOMICAL ALMANAC, PP. 103-104, AND CAPITAINE ET AL. (2003), ASTRONOMY AND ASTROPHYSICS 412, 567-586.

THIS SUBROUTINE APPLIES PROPER MOTION, INCLUDING FORESHORTENING EFFECTS, TO A STAR'S POSITION.

THIS SUBROUTINE PREDICTS THE RADIAL VELOCITY OF THE OBSERVED OBJECT AS IT WOULD BE MEASURED BY SPECTROSCOPIC MEANS. RADIAL VELOCITY IS HERE DEFINED AS THE RADIAL VELOCITY MEASURE (Z) TIMES THE SPEED OF LIGHT. FOR A SOLAR SYSTEM BODY, IT APPLIES TO A FICTITIOUS EMITTER AT THE CENTER OF THE OBSERVED OBJECT, ASSUMED MASSLESS (NO GRAVITATIONAL RED SHIFT), AND DOES NOT IN GENERAL APPLY TO REFLECTED LIGHT. FOR STARS, IT INCLUDES ALL EFFECTS, SUCH AS GRAVITATIONAL RED SHIFT, CONTAINED IN THE CATALOG BARYCENTRIC RADIAL VELOCITY MEASURE, A SCALAR DERIVED FROM SPECTROSCOPY. NEARBY STARS WITH A KNOWN KINEMATIC VELOCITY VECTOR (OBTAINED INDEPENDENTLY OF SPECTROSCOPY) CAN BE TREATED LIKE SOLAR SYSTEM OBJECTS. SEE LINDEGREN & DRAVINS (2003), ASTRONOMY & ASTROPHYSICS 401, 1185-1201.

THIS SUBROUTINE COMPUTES ATMOSPHERIC REFRACTION IN ZENITH DISTANCE. THIS VERSION COMPUTES APPROXIMATE REFRACTION FOR OPTICAL WAVELENGTHS. IT CAN BE USED FOR PLANNING OBSERVATIONS OR TELESCOPE POINTING, BUT SHOULD NOT BE USED FOR THE REDUCTION OF PRECISE OBSERVATIONS. BASIC ALGORITHM IS DESCRIBED IN THE EXPLANATORY SUPPLEMENT TO THE ASTRONOMICAL ALMANAC, P. 144, AND IS AN ADAPTATION OF A FORMULA IN BENNETT (1982), JOURNAL OF NAVIGATION (ROYAL INSTITUTE) 35, 255-259.

THIS SUBROUTINE ALLOWS FOR THE SPECIFICATION OF THE DELTA-T VALUE (DELTA-T = TT - UT1) TO BE USED IN THE CALCULATION OF SIDEREAL TIME AND THE TERRESTRIAL-TO-CELESTIAL TRANSFORMATION. IT ALLOWS THESE CALCULATIONS TO BE PERFORMED, CORRECTLY, USING UT1 AS THE TIME ARGUMENT FOR THE EARTH ROTATION ANGLE AND TDB AS THE TIME ARGUMENT FOR THE PRECESSION AND NUTATION COMPONENTS. THIS SUBROUTINE, IF USED, SHOULD BE CALLED BEFORE ANY SUBROUTINE RELATED TO EARTH ROTATION (E.G., SIDTIM OR TERCEL) FOR A GIVEN DATE. THE VALUE OF DELTA-T SPECIFIED HERE WILL BE USED UNTIL EXPLICITLY CHANGED.

THIS SUBROUTINE ALLOWS THE USER TO SPECIFY THE 'MODE' VALUE, WHICH DETERMINES THE METHOD USED FOR THE COMPUTATION OF SIDEREAL TIME AND THE TERRESTRIAL-TO-CELESTIAL TRANSFORMATION, AND THE ACCURACY OF NUTATION AND RELATED CALCULATIONS.

THIS SUBROUTINE COMPUTES THE GREENWICH SIDEREAL TIME (EITHER MEAN OR APPARENT) AT JULIAN DATE TJDH + TJDL.

SUBROUTINE SOLSYS VERSION 3. THIS SUBROUTINE PROVIDES THE POSITION AND VELOCITY OF THE EARTH AT EPOCH TJD BY EVALUATING A CLOSED-FORM THEORY WITHOUT REFERENCE TO AN EXTERNAL FILE. THIS ROUTINE CAN ALSO PROVIDE THE POSITION AND VELOCITY OF THE SUN.

SUBROUTINE SOLSYS VERSION 1. THIS SUBROUTINE READS A COORDINATE FILE CONTAINING BARYCENTRIC POSITIONS OF SOLAR SYSTEM BODIES AT DAILY INTERVALS AND PROVIDES THE POSITION AND VELOCITY OF BODY M AT EPOCH TJD.

THIS SUBROUTINE TRANSFORMS A VECTOR FROM ONE COORDINATE SYSTEM TO ANOTHER WITH SAME ORIGIN AND AXES ROTATED ABOUT THE Z AXIS.

FOR USE WITH SUBROUTINE SOLSYS VERSION 3. THIS SUBROUTINE COMPUTES THE COORDINATES OF THE EARTH-SUN POSITION VECTOR WITH RESPECT TO THE ECLIPTIC AND EQUATOR OF DATE. A MODIFIED FORM OF NEWCOMB'S THEORY ('TABLES OF THE SUN', 1898) IS USED. ONLY THE LARGEST PERIODIC PERTURBATIONS ARE EVALUATED, AND VAN FLANDERN'S EXPRESSIONS FOR THE FUNDAMENTAL ARGUMENTS ('IMPROVED MEAN ELEMENTS FOR THE EARTH AND MOON', 1981) ARE USED. THE ABSOLUTE ACCURACY IS NO WORSE THAN 1 ARCSECOND (AVERAGE ERROR ABOUT 0.2 ARCSECOND) OVER 1800-2200. (ADAPTED FROM SUBROUTINE IAUSUN BY P. M. JANICZEK, USNO.)

THIS SUBROUTINE ROTATES A VECTOR FROM THE TERRESTRIAL TO THE CELESTIAL SYSTEM. SPECIFICALLY, IT TRANSFORMS A VECTOR IN THE ITRS (A ROTATING EARTH-FIXED SYSTEM) TO THE GCRS (A LOCAL SPACE-FIXED SYSTEM) BY APPLYING ROTATIONS FOR POLAR MOTION, EARTH ROTATION, NUTATION, PRECESSION, AND THE DYNAMICAL-TO-GCRS FRAME TIE.

THIS SUBROUTINE COMPUTES THE POSITION AND VELOCITY VECTORS OF A TERRESTRIAL OBSERVER WITH RESPECT TO THE GEOCENTER.

THIS SUBROUTINE COMPUTES THE TERRESTRIAL TIME (TT) JULIAN DATE CORRESPONDING TO A BARYCENTRIC DYNAMICAL TIME (TDB) JULIAN DATE. THE EXPRESSION USED IN THIS VERSION IS A TRUNCATED FORM OF A LONGER AND MORE PRECISE SERIES GIVEN BY FAIRHEAD & BRETAGNON (1990) A&A 229, 240. THE RESULT IS GOOD TO ABOUT 10 MICROSECONDS.

THIS SUBROUTINE CONVERTS ANGULAR QUANTITIES RELATED TO A STAR'S POSITION AND MOTION TO VECTORS.

THIS SUBROUTINE CORRECTS A VECTOR IN THE ITRS (A ROTATING EARTH- FIXED SYSTEM) FOR POLAR MOTION, AND ALSO CORRECTS THE LONGITUDE ORIGIN (BY A TINY AMOUNT) TO THE TERRESTRIAL INTERMEDIATE ORIGIN (TIO). THE ITRS VECTOR IS THEREBY TRANSFORMED TO THE TERRESTRIAL INTERMEDIATE SYSTEM, BASED ON THE TRUE (ROTATIONAL) EQUATOR AND THE TERRESTRIAL INTERMEDIATE ORIGIN (TIO). SINCE THE TRUE EQUATOR IS THE PLANE ORTHOGONAL TO THE DIRECTION OF THE CELESTIAL INTERMEDIATE POLE (CIP), THE COMPONENTS OF THE OUTPUT VECTOR ARE REFERRED TO Z AND X AXES TOWARD THE CIP AND TIO, RESPECTIVELY.

THIS SUBROUTINE TRANSFORMS TOPOCENTRIC RIGHT ASCENSION AND DECLINATION TO ZENITH DISTANCE AND AZIMUTH. THIS ROUTINE USES A METHOD THAT PROPERLY ACCOUNTS FOR POLAR MOTION, WHICH IS SIGNIFICANT AT THE SUB-ARCSECOND LEVEL. THIS SUBROUTINE CAN ALSO ADJUST COORDINATES FOR ATMOSPHERIC REFRACTION.